(2) f(x)=2x-x^2
x>1单减
X∈[a,b]时,g(x)=f(x)且g(x)的值域为[1/b 1/a]
g(a)=1/a g(b)=1/b
2a-a^2=1/a
a^3-a^2-a^2+a-a+1=0
(a-1)(a^2-a-1)=0
a1=1 a2=(1+根号5)/2 a3直接舍去
g(a)=1/b g(b)=1/a (a